The invention relates to an apparatus for investigating the reactants involved in a reaction between a gas and a solid by means of X-ray absorption spectroscopy (XAS). The invention is also concerned with an XAS-method for investigating gas-solid reactions in situ using this apparatus.
For the investigation of gas-solid reactions and their reaction products it is frequently necessary to use spectroscopic techniques in which the intrusion of the probe and the disturbance of the reaction environment during the investigation is reduced to a minimum. This applies in particular to investigations in the field of heterogeneous catalysts. Here, in situ investigations are of particular significance. It has been established that ex situ or post mortem investigations which are not carried out under working conditions yield somewhat different results as compared with in situ investigations.
An apparatus and a method for the in situ investigation of catalytic agents by means of X-ray absorption spectroscopy is known for example from "Nuclear Instruments and Methods in Physics Research" B97 (1995) pages 28-32. The apparatus described there comprises a measuring cell with a sample holder for solid samples located in the cell and with a window for the entry of X-rays for the irradiation of the sample. The sample holder consists of a gold-coated plate of silica (silicon dioxide) which is arranged within a cylinder of the same material which is open at both ends. This cylinder, whose axis lies parallel to the direction of propagation of the X-rays, is biassed to a positive or negative potential with reference to the sample holder and serves as a collector arrangement for the collection of electrons which are emitted by the sample which is fixed to the sample holder as a result of the X-ray absorption. The electrical current which flows from the collector electrode to a ground connection is used as a measurement value for the plotting of the absorption spectrum. The principle belongs to the field of total electron yield detection (TEY detection), since only the total yield of electrons is determined, without analysis of their kinetic energy. Values &gt;20V have been established as preferable for the potential difference between collector arrangement and sample holder. Investigations at pressures up to 1 atmosphere and at temperatures up to 500.degree. C. in the range of middle and hard X-radiation (.gtoreq.4.5 keV) have been carried out on samples such as methanol synthesis catalysts Cu/ZnO/Al.sub.2 O.sub.3, a copper/nickel alloy as well as a nickel reforming catalyst.
Having regard to the aforementioned publication, further investigations are reported in "Faraday Discussions" 105 (1996) pages 317-336. There it is maintained that the TEY detection uses the proportionality between the absorption coefficient of the sample for X-ray radiation and the number of the Auger electrons emitted from the sample, with the TEY signal having a linear relationship to the absorption coefficient of the sample. In the book "Practical Surface Analysis" by D. Briggs and others (John Wiley, New York 1983) it had already been established that the average penetration depth of the most energetic Auger electrons from the sample determines the surface sensitivity, and that as a consequence the surface sensitivity of the TEY signal depends upon the energy of the absorption edge. It has been observed that the average penetration depth of a few nm for absorption edges with &lt;1 keV changes to several hundred nm for X-ray energies &gt;10 keV.